This invention relates to a method for the high speed measurement of the frequency response of a linear system, and more particularly to frequency response measurement in which a binary pseudo-random noise sequence is applied to the linear system and the power spectra of the output is determined directly by Fast Fourier Transform (FFT) techniques.
The magnitude of the transfer function or amplitude of the frequency response of a linear system is an important parameter in system design. Because it is the fundamental specification in many electrical and mechanical systems, such as filters, amplifiers, radio receivers, and vibrating mechanical equipment, its determination from the physical system is important for both diagnostic and analytical reasons. There are two basic methods for accomplishing this measurement. One technique is to inject successive sinusoidal excitations at different frequencies into the system and measure the magnitude of the response of each. Another technique involves applying random inputs to the system and analyzing the random output, and includes injecting white noise into the system under test and analyzing the output with a narrow filter band set. Both of these techniques have disadvantages if the measurement is to be made rapidly with a high degree of frequency resolution. In the former, the system must reach steady state for each excitation, and many measurements must be made to achieve good frequency resolution. In the latter, sufficient averaging time is required to obtain enough certainty in the measurement because the output is statistically random, and a tight filter set is needed for the specific method given.
Although either of the foregoing techniques can supply a reliable, high resolution measurement of the frequency response of an unknown system being tested, the time required to make the measurement is often excessive. This problem arises if the measurement is to be made on a high speed production line as a part of a quality assurance check or if the frequency response of the system is changing with time.